The direct fired downhole steam generator disclosed and claimed in my above-mentioned co-pending application has found substantial use and has provided satisfactory and efficient thermal stimulation of existing oil wells, particularly where the sands subjected to "steam drive" are located at depths greater than 2,000 feet from the surface. However, there are a large number of wells wherein surface generated steam can be efficiently utilized.
As indicated in the above-mentioned co-pending application however, state of the art conventional steam generators or boilers operating on the earth's surface or abovehole, inherently produce substantial amounts of combustion or "stack gases" due to the nature of the combustion process employed. With these boilers, products of combustion cannot be prevented from entering the atmosphere. The obvious environmental impact of any such large scale combustion is highly undesirable and, in fact, has limited the use of surface steam generation by boilers in many areas where atmospheric pollution is critical.
Known direct contact steam generators operating at near atmospheric pressure require extremely large combustion chambers, in order to provide adequate heat exchange to the particular liquid being heated. Additionally, these units suffer and/or include shortcomings of both direct and indirect steam generation, in that due to the large areas of feedwater exposed to the combustion chamber, substantial amounts of combustion products are absorbed or dissolved into the heated water. However, since most of the combustion gas volume is not absorbed, substantial stack or exhaust gases must be vented to the atmosphere resulting in the aforementioned environmental problems.
Direct injections of both steam and combustion gases to enhance oil recovery has been shown to be more effective in thermal stimulation of the wells, since there is evidence to the effect that combustion gases are soluble and retained in crude oil, causing an increase in volume, thereby enhancing release from associated oil sand. High pressure combustion utilized in the direct steam generator of the system disclosed herein, provides increased thermal capacity for a given size, resulting in an equipment package greatly reduced in size.
Small generator size provides an additional advantage in the area of safety, since actual volume of generated steam within the generator at any given time is exceedingly small, greatly reducing the possibility of damage in the case of a generator failure.
Both downhole steam drive and surface generated steam drive however, suffer from the common economic problem of high fuel consumption due to the relatively large amount of heat required to thermally stimulate oil sands. A generally accepted figure within the industry is that approximately 30% (thirty percent) of the thermal energy recovered in stimulated production is returned or lost in the stimulation process. Fuel costs involved in thermal stimulation makes it exceptionally attractive for operators of steam drive equipment to utilize the locally available fuels such as leased crude, "heavy" oil, i.e. Bunker C or equivalent, or other carbonaceous material such as coal, sawdust, or other organic waste material.
As discussed above, conventional surface steam generators, particularly when fired with low cost fuels, emit substantial and objectionable combustion gases. This problem limits the use of fuels such as residual oil, leased crude oil, and other carbonaceous fuels in state of the art equipment. Further, both downhole and abovehole generating equipment, require that the combustion process must be essentially "clean", since injected steam and combustion products cannot be allowed to contaminate the oil sands they are required to stimulate.
Applicant's invention overcomes these difficulties through the use of high pressure combustion techniques, wherein the combustion process heats feedwater and generates steam after the combustion process is complete. A primary feature of the approach disclosed herein is a means for employing a high pressure combustor in order to utilize less desirable fuels known to generate undesirable atmospheric pollutants.
In keeping with the invention, undesirable material attendant to the combustion process are effectively removed from the generator output, providing a steam/combustion gas mixture which can be directly injected downhole for effective thermal stimulation.